Design Of The Solar Heating System Thermal Storage Devices Based On Heat Flow Fluctuation Characteristics

Heat storage device is the key component to solve the mismatch of time between heat supply and demand,that is,it can realize the transfer of heat in time,but also change the fluctuation characteristics of working temperature.In this thesis,the dynamic response relationship between the heat flow fluctuation characteristics of the heat source side and load side of the heat storage tank with the output of the heat supply side was studied,to guide the design,optimization and thermal performance test of the heat storage tank.The relationship between the heat flow fluctuation characteristics of the heat source side and load side and the thermal performance of the heat storage device was revealed.In this thesis,the computational fluid dynamics method（CFD）was taken,the heat flow fluctuation at the heat source side and the load side was simplified to sine wave.Based on frequency domain analysis method,the response characteristics and thermal performance of the output end of the heat storage tank were studied from the perspective of frequency domain under the conditions of different heat collecting power fluctuation（amplitude/period）,heat load fluctuation（amplitude/period）,phase difference between fluctuations,layered design and water tank volume,the optimal design method of solar heating storage device was put forward.Frequency domain analysis results of outlet temperature of heating side showed that,after the fourier transform,the phenomenon that the amplitude of the components at one or more frequencies was much greater than that of the others existed.The sinusoidal wave corresponding to the maximum amplitude could be used to reflect the fluctuation characteristics of heating output temperature.The fluctuation period of the outlet temperature of the heating side was only determined by the fluctuation period of external heat flow,and had nothing to do with other factors.The amplitude increased with the increase of the amplitude of heat collection power/heat load,the decrease of period gap,the phase difference approachingπ,good thermal stratification,and the decrease of tank volume.The amplitude attenuation rate(A_y1/A_Y1)varied from 0.4 to 1 on the heat source side,and the phase difference of heat collection power/heat load had the most obvious influence on the attenuation rate.The phase was mainly affected by heat collection power/the phase of heat load fluctuation and the volume of water tank,the hysteresis phase angle（0.23rad～0.81rad）relative to the temperature wave on the heat source side was most affected by the volume of the tank.The wave constant term increased with the increase of amplitude of heat collection power/heat load,the decrease of period gap,the phase difference approachingπ,good thermal stratification,and the decrease of tank volume.The difference with temperature wave constant on the heat source side was mainly affected by the stratification and volume of the water tank.The amplitude,phase and period of the temperature at the heating outlet could directly reflect the thermal performance of the heat storage tank（effective heat storage、exergic efficiency and efficiency of heat storage and release）.The thermal performance analysis results showed,the amplitude of outlet temperature T₂ on the heating side and the fluctuation reference temperature could affect the efficiency of heat storage and release,effective heat storage,exergic efficiency;the phase could change the heat storage and release efficiency,effective heat storage fluctuations;The efficiency of heat storage and release varied from 96.7%to 99.3%,the maximum effective heat storage increased from 15.01 k Wh to 65.89 k Wh,exergic efficiency remained at 78.3%to 94.3%;The phase could change the instantaneous heat storage and release efficiency and the change trend of effective heat storage with time.The period could affect the instantaneous efficiency of heat storage and release and the fluctuation period of effective heat storage.The influence of outlet temperature fluctuation characteristics of the same heating side on the thermal performance of different water tank structures was the same,but the influence degree was inconsistent.The influence of different heat flow wave characteristics on thermal performance of heat storage tank was studied,the optimization objective was to make more effective use of the heat energy inside the heat storage tank and improve the matching degree between the heat supply output temperature of the heat storage tank and the demand temperature of the load side,based on the dynamic performance of heat storage tank,a new design method of heat storage tank was proposed.In this case,the optimized heating output temperature range of the system at the peak load stage was346.88-352.74 K,the average temperature was 4 K higher than that of the original system,the volume of the water tank was reduced by 0.53 m³,and the total heat input of the optimized system was reduced by 0.72 k Wh,but the heat storage and release efficiency was increased by 2%.The peak period of heat supply temperature fluctuation was more matched with the peak period of heat load demand.